1. Field of the Invention
The present invention relates to a gyroscope used in a navigation system or the like and a drive detection device therefor and, more particularly, to a drive detection device for a gyroscope using a piezoelectric vibrator which can be easily manufactured by achieving a reduction of the number of electrodes and simplification of dielectric polarization.
2. Description of the Related Art
FIG. 4 is a perspective view showing a trident-type tuning fork piezoelectric vibrator as an example of a conventional vibratory gyroscope, and is of the same type as that of, e.g., a piezoelectric vibrator disclosed in Japanese Unexamined Patent Publication No. 9-101156. FIG. 5A is a front view obtained by viewing the piezoelectric vibrator shown in FIG. 4 in the direction of arrow V, and FIG. 5B is a front view showing a drive state.
In the piezoelectric vibrator shown in FIG. 4, three vibrators parallel separated from each other are formed at the distal end of an elastic plate entirely made of a piezoelectric material such as piezoelectric ceramic. In this piezoelectric vibrator, since the vibrators on both the sides vibrate in the same phase, the vibrators on both the sides are indicated by the same reference numeral 1. Since the middle vibrator vibrates in a phase different from the phases of the vibrators on both the sides, the middle vibrator is indicated by reference numeral 2 which is different from the reference numerals of the vibrators 1 on both the sides.
As shown in FIGS. 5A and 5B, electrodes 5a, 5b, and 5c are formed on front surfaces 1a of the left and right vibrators 1 on both the sides, and electrodes 6a, 6b, and 6c are formed on rear surfaces 1b. Electrodes 7a, 7b, and 7c are formed on a front surface 2a of the middle vibrator 2, and electrodes 8a, 8b, and 8c are formed on a rear surface 2b. As shown in FIG. 4, the respective electrodes extend along the direction of axis Z throughout the entire lengths of the vibrators 1 and 2 in the longitudinal direction.
The vibratory drive directions of the vibrators 1 and 2 is X directions (first directions). In the vibrators 1 and 2, when the X directions (first directions) which are vibratory drive directions are set to be directions of width, the electrodes 5a, 5c, 6a, 6c, 7a, 7c, 8a, and 8c are formed at both the edge portions of the vibrators 1 and 2 in the directions of width. The electrodes 5b, 6b, 7b, and 8b are located at the centers of the vibrators 1 and 2 in the directions of width (X directions).
FIG. 5A shows polarities of an applied electric field when dielectric polarization is performed to a piezoelectric material. DC voltages applied to respective electrodes are represented by + and -, and a ground potential is represented by G. In the vibrators 1 on both the sides, the electrodes 5b and 6b located at the centers in the directions of width on the front and rear surfaces have ground potentials. On the front surface 1a, a negative voltage is applied to the electrodes 5a and 5c located at both the edge portions in the directions of width. On the rear surface 1b, a positive voltage is applied to the electrodes 6a and 6c located at the edge portions in the directions of width. In the middle vibrator 2, the electrodes 7b and 8b located at the centers have ground potentials. On the front surface 2a, a negative voltage is applied to the electrodes 7a and 7c located at both the edge portions in the directions of width. On the rear surface 2b, a positive voltage is applied to the electrodes 8a and 8c located at both the edges in the directions of width. Arrows shown in FIG. 5A are directions of electric fields applied across the electrodes at this time, and dielectric polarization is performed along the electric field directions.
In this piezoelectric vibrator, the electrodes 6b and 8b of the vibrators 1 and 2 are used as detection electrodes. The detection electrodes 6b and 8b are formed on surfaces (1b, 2b) extending the X directions (first directions) on the vibrators 1 and 2 and formed at the central positions in the directions of width of the X directions. In the vibrator 1, dielectric polarization directions on the left and right of the X directions are symmetrical with respect to the portion of the detection electrode 6b. Similarly, in the vibrator 2, dielectric polarization directions are symmetrical on the left and right of the X directions with respect to a portion where the detection electrode 8b.
In FIG. 5B, the phases of AC drive voltages applied to the respective electrodes are represented by signs + and -. When sign + is expressed on a certain electrode, and sign - is expressed on the other electrode, it means that AC drive voltages having a phase difference of 180.degree. (.pi.) are applied to both the electrodes. Mark o in FIG. 5B represents plus distortion (extension) caused by the piezoelectric effect, and mark x represents minus distortion (contraction).
In the drive method in FIG. 5B, the electrodes 5b, 6a, 6c, 7b, 8a, and 8c are grounded. The electrodes 5a, 5c, 7a, and 7c are drive electrodes located on the front surfaces 1a and 2a of the vibrators 1 and 2, and the electrodes 6b and 8b located at the centers of the rear surfaces 1b and 2b.
As an AC drive power, voltages which are in-phase are applied to the electrodes 5c and 7a, and voltages which are in-phase (opposite from the above phase) are applied to the electrodes 5a and 7c. As a result, on the surfaces 1a of the vibrators on the left and right, at a certain point of time, plus distortion o occurs between the electrodes 5a and 5b, and minus distortion x occurs between the electrodes 5b and 5c. In the middle vibrator 2, on the front surface 2a, minus distortion x occurs between the electrodes 7a and 7b, and plus distortion o occurs between the electrodes 7b and 7c. Therefore, at a certain point of time shown in FIG. 5B, bending vibration occurs such that the amplitude directions of the vibrators 1 on both the sides are set to be a +X direction, and the amplitude of the middle vibrator 2 is performed in a -X direction. More specifically, the vibrators 1 on both the sides and the middle vibrator 2 vibrate with phases opposite from each other in the X directions.
When the piezoelectric vibrator is placed in a rotation system rotated about axis Z, force in Y directions (second directions) which are orthogonal to the vibration direction works due to Coriolis force. Since the left and right vibrators 1 and the middle vibrator 2 are vibratorily driven with phases opposite from each other in the X directions, the vibration components generated by Coriolis force are opposite from each other in phase in the vibrators 1 on both the sides and the middle vibrator 2. For example, when the amplitude direction of the vibrators 1 on both the sides is a +Y direction at a certain point of time, the amplitude direction of the middle vibrator 2 is a -Y direction.
Vibration components generated by the Coriolis force are obtained from the detection electrodes 6b and 8b formed at the centers of the rear surfaces 1b and 2b of the vibrators 1 and 2 in the directions of width.
In the vibration component of each vibrator generated by Coriolis force, when the amplitude direction of the vibrators 1 is the +Y direction at a certain point of time, the piezoelectric materials of the portions of the detection electrodes 6b "extends". The amplitude direction of the middle vibrator 2 becomes the -Y direction, and the piezoelectric material of the portion of the detection electrode 8b "contracts". Since all the dielectric polarization directions of the portions where the detection electrodes 6b, 8b, and 6b are formed are equal to each other, current outputs I1 and I2 of the detection electrodes 6b are in-phase as the vibration components of the vibrators in the Y directions. In contrast to this, a current output I3 of the detection electrode 8b is detected to have a phase opposite from the phase of the current outputs I1 and I2.
However, in the conventional piezoelectric vibrator, a pair of drive electrodes 5a and 5c, a pair of drive electrodes 7a and 7c, output electrodes 6a and 6c, and output electrodes 8a and 8c are formed on the front surfaces 1a and 2a and the rear surfaces 1b and 2b of the vibrators 1 and 2, respectively, and the ground electrodes 5b and 7b and the detection electrodes 6b and 8b are formed between the pair of drive electrodes and between the pair of drive electrodes, respectively, and six electrodes must be formed on both the front and rear surfaces of one vibrator. Therefore, dielectric polarization and steps in manufacturing the electrodes are cumbersome, and wires connected to the electrodes are complex.
The electrodes formed on the front surfaces 1a and 2a and the rear surfaces 1b and 2b of the vibrators have a structure in which the electrodes are very close to each other to have a small interval size (creeping distance).
As shown in FIG. 5A, dielectric polarization in each vibrator is generated by applying a high voltage across the electrodes. However, when the interval size between the electrodes is small as described above, and the shapes of the electrodes are not uniform, the following problems are posed. That is, discharge caused by dielectric breakdown is generated at the nonuniform portion when a high voltage is applied, and the electrodes are broken. When dielectric polarization is generated at a low voltage to prevent the electrodes from being broken, the problem that the dielectric polarization is not sufficiently performed is posed.
More specifically, due to a tolerance generated when the electrodes are formed, the electrodes cannot be avoided from being formed and biased in any one direction of the X directions, it is difficult to make the dielectric polarizations on both the sides of each electrode completely symmetrical. Therefore, a direction in which a detection electrode is biased by an error in manufacturing cannot be predicted, and an error of the symmetry of the dielectric polarization directions cannot be also predicted.
Therefore, polarities of the vibration components output from the detection electrodes in the X directions are random, the polarities with which the vibration components are generated cannot be predicted. For this reason, when the current outputs I1, I2, and I3 from the detection electrodes are given to the detection circuit described above, the current outputs may be amplified by adding the vibration components in the X directions, or may be decreased by subtracting the vibration components. More specifically, when the positions at which the detection electrodes are formed have errors, amounts of vibration component in the X directions included in the detection output cannot be predicted until the piezoelectric vibrator is actually operated. For this reason, the detection accuracy of Coriolis force becomes low, and sensitivity for detecting an angular velocity X about axis Z is low.